<title>Link Lists</title>
<body bgcolor="#ffffcc">
<hr>
<center> <h1>Link Lists</h1> </center>
<hr>
<p>
<ul>
<li><a href="#long">Long explanation</a>
<li><a href="#short">Short explanation</a>
</ul>

<a name=long>
Question, how would you write a program to meet the following requirements?

<ol>
<li>Read an unknown number of records from a file into memory. A typical
record would look like:
<pre>
	Ref  Title	  Cost
	---  -----	  ----
	1234 Oracle_Guide 22.95
</pre>
<li>Perform a numeric sort based on the first field.
<li>Delete duplicate records based on the first field.
</ol>

One method is to define an 
<a href="../CONCEPT/arrays.html#chard">array</a> of records as below:

<pre>
	main()
 	{
	  char array[50][80]; /* 50 records, 80 bytes long	*/
	}
</pre>

The data can then be read into the array and then actions performed 
on the data.
This is OK but has some major problems.
<ol>
<li>The arrary will hold all the data in character format. It would be 
nice to hold the integer and decimal numbers in a more appropriate form.
<li>If you have more than 50 records the program has to be altered
and recompiled.
</ol>
The first problem could be fixed by defining an 
<a href="../SYNTAX/struct.html#array">array of structures</a> BUT both problems 
can be solved with linked lists.<p>

<a name=short>

<img src=../../GRAPHICS/linklist.gif align=right>
<p>
The concept of a linked list is fairly simple. It is a group of structures
linked together by pointers, here is a picture:<p>
<br clear=right>
<p>
<p>
The "Name Age Pointer" block could be defined as below:
<pre>
	struct record {char name[20]; int age; struct record *next_rec;}; 
</pre>

The important bit is "struct record *next_rec" this is the pointer to the next
structure (record). It will either point to the next record which 
will require memory reserved 
with the <a href="../FUNCTIONS/malloc.html">malloc</a> function or
<a href="../SYNTAX/null.html">NULL</a> if  its the last record.
<p>

<h2>Examples</h2>

<img src=../../GRAPHICS/computer.gif align=left>
<a href="../EXAMPLES/linklst1.c"> Here is the first example.</a> 
This program is more complicated than I would 
like, but its the simplest I can come up with. 
<br clear=left>
<p>
<img src=../../GRAPHICS/computer.gif align=center>
<a href="../EXAMPLES/linklst2.c">
This is the same program</a> but without the heavy commenting.
<br clear=left>
<p>
<img src=../../GRAPHICS/computer.gif align=center>
<a href="../EXAMPLES/linklst3.c">
This is still the same program</a> but it is slightly simplified with the use 
of <a href="../SYNTAX/typedef.html">typedef</a>.
<br clear=left>

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<address><a href="../../address.html">Martin Leslie</a> 
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